What is the static pressure difference between inside and outside airframe?

[FORANE]

I currently have an old Altrak altitude hold in the Lancair. When I initially installed it, I connected it to the static port. After experiencing a frozen static line one time I removed the Altrak's connection to the static port. I figured it was just an altitude hold and didn't matter what altitude that might be; removing the static port increased its safety because if I were in IMC and the static line froze again, at least the altitude hold would keep me level.

I am currently installing a Trio pro pilot in the RV-9A I recently purchased. I am debating if I want to connect it to the static port.

On the ground the static line pressure should be the same as inside the cabin. In flight I seem to recall there is a decrease in cabin pressure due to the venturi effect. As such, the pro pilot would level off lower than actual altitude selected due to the error.

My question is does anyone know how much the error may be? I presume the error may vary between aircraft and of course both the aircraft in question are not pressurized.

 

[flyingron]

Depends on the geometry of the aircraft, but most aircraft are lower in pressure than ambient. This is why alternate static has errors, etc...

 

[poadeleted20]

The error can be determined only through flight testing throughout the flight envelope, as the difference varies with speed and other factors including cabin heating and ventilation system status. That's why airplanes with alternate static sources have correction data in their POH, and restrictions such as cabin heat on and cabin ventilation off to minimize the errors. For those reasons, I'm pretty sure the FAA does not allow production aircraft to do what you want, and probably has some serious operating limitations if you do it on an E-AB -- EAA would be the right place to get answers on that. I also suspect that if you do that, you're going to get some serious wandering in altitude hold and VS hold modes, as well as significant inaccuracies in altitude preselect mode, and that those errors will vary depending on speed and heating/ventilation settings. All in all, even if it's legal, I think you'll be very unhappy with the result.

 

[FORANE]

Thanks for the feedback guys.
I believe I am going to plumb the autopilot into the static line and add an alternate static air source in the panel.

 

[RV10flyer]

My -10 dives about 200-250' when alternate static valve is opened with cabin heat and fresh air vents closed. When alt static valve is opened with front/rear heat, four fresh air vents already opened...no noticeable change.

 

[poadeleted20]

Thanks for the feedback guys.
I believe I am going to plumb the autopilot into the static line and add an alternate static air source in the panel.

Sounds like a good plan to me.